A conventional pneumatic radial tire, especially an aircraft radial tire has a large swell of a tread surface in its radial direction by a high internal pressure and a centrifugal force caused by high speed rotation.
If the tread surface swells in the radial direction, a tread rubber is expanded in a circumferential direction of the tire.
Generally, the aircraft radial tire is used under conditions of high internal pressure and high load. Therefore, when the tire rides over foreign matter, the aircraft radial tire is poor in properties by which a tread thereof is not damaged when the entire tire rides over the foreign matter, i.e., so-called “enveloping properties”. In a state in which the tread rubber of the tire is expanded in the circumferential direction of the tire, a resistance force against the foreign matter becomes weak, and there is a problem that the trampled foreign matter easily enters the tread, and the tire is prone to be damaged.
When an amount of swell of the central portion of the tire in its widthwise direction becomes greater than opposite ends of the tire in the widthwise direction, a diameter difference is generated. This diameter difference causes a drag phenomenon to the rotating tire, a shoulder portion is worn earlier than the central portion of the tire, and the lifetime of the tire is shortened. This phenomenon is called a deviated wear.
In a conventional radial tire using a relatively low elastic cord as a belt layer a width of a main portion of a crown reinforcing ply layer is equal to a ground-contacting width, and belt layers which are adjacent to each other inner side and outer side in the radial direction of the tire are laminated on one another with extremely small steps interposed therebetween in most of cases (see JP-A No. 5-193306).
In order to improve the wearing characteristics of the tread by suppressing the swelling deformation thereof, to enhance the wearing characteristics of the tread, and to enhance the enveloping properties, a pneumatic radial tire 102 shown in a transverse cross section in FIG. 22 is considered. In the pneumatic radial tire 102, a belt layer 20 disposed between a tread rubber layer 24 and a crown region of a carcass layer 16 comprises a conventional main belt layer 26 comprising wide belt ply, and an auxiliary belt layer 28 which is added on an outer periphery of the main belt layer 26 and which comprises narrow belt ply. With this structure, the belt rigidity is enhanced by the auxiliary belt layer 28 disposed on a central portion of the main belt layer 26, and especially the swelling deformation of the tread central region is restrained.
In order to suppress the swell caused by the internal pressure of the central portion in the widthwise direction of the tire, in a conventional simple method, a ply cord is replaced by a cord which has relatively high elasticity and which is made of aromatic polyamide (see Japanese Patent Application Laid-open (JP-A) No. 61-178204).
As compared with an aliphatic polyamide which is conventionally used for an aircraft tire, the aromatic polyamide cord exhibits high tension also in a low elongation percentage region and maintains the internal pressure. Therefore, the swell of the tire can effectively be suppressed.
As a conventional technique for enhancing an anti-cut separation performance, a pneumatic radial tire in which a narrow reinforcing layer (cord is made of steel) is additionally disposed in the belt is proposed (see JP-A No. 8-58310).
As another conventional technique, there is proposed a pneumatic radial tire in which a reinforcing layer comprising cords which are reinforced by strong cords such as glass, metal, aramid or the like is provided on an outermost layer of a belt made of organic fiber (see U.S. Pat. No. 4,216,813).
In these conventional techniques, a belt having higher tension is further added.
According to the conventional technique as shown in FIG. 22, however, by securing a necessary rubber thickness H (thickness of rubber only and thickness of cord is not included) in the tread central region where the total thickness of the belt layer becomes most thick, there are problems that a rubber thickness H0 of the tread side region becomes excessively thick and thus, the tire weight is increased forcibly, a heating value of the tread side region is increased and thus, the high speed endurance is lowered.
In the conventional belt structure, only by using a relatively high elastic cord in the belt ply, performance in which the cord characteristics are sufficiently exhibited can not be obtained, and it is difficult to reduce the weight of members to be used as light as possible.
First of all, the number of belts of an aircraft tire is high, and it is not preferable to further increase the number of belts, and since a centrifugal force is applied when the tire rotates, it is preferable to reduce the weight of the tread portion.
Thus, in order to meet these requirements, it is absolutely necessary to employ a belt disposition without waste and a structure in which characteristics of the cord can be utilized.
In view of the above circumstances, it is an object of the invention to provide a pneumatic radial tire suitable for an aircraft in which a diameter of the tread surface is prevented from being increased, endurance against cutting occurred by foreign matter or the like is enhanced, and the weight of the tire is reduced.